diff --git a/scripts/library_run.py b/scripts/library_run.py
index 79618f8..82c83f9 100644
--- a/scripts/library_run.py
+++ b/scripts/library_run.py
@@ -82,6 +82,13 @@ def pin_child(pid):
 def run_pinned(cmd, timeout=None, **kw):
     """subprocess.run() with the child pinned to CHILD_MASK (see pin_child)."""
     kw.setdefault("creationflags", NOWIN)
+    # `capture_output` is a subprocess.run() convenience Popen does not accept - expand it here,
+    # otherwise every caller that passes it dies with
+    # "Popen.__init__() got an unexpected keyword argument 'capture_output'" (measured: all three
+    # lanes failed every item within a second of this wrapper being introduced).
+    if kw.pop("capture_output", False):
+        kw.setdefault("stdout", subprocess.PIPE)
+        kw.setdefault("stderr", subprocess.PIPE)
     p = subprocess.Popen(cmd, **kw)
     pin_child(p.pid)
     try: